Condensation on the Inside of Windows: Why It Happens and How to Stop It

Water beaded on the glass every winter morning, puddles on the sill, and frost on the inside of the pane have one cause: warm, humid indoor air meeting glass chilled below the air's dew point. Andersen puts it plainly: condensation forms inside the window because the humidity indoors is too high, not because something is wrong with the window. Pella agrees, and adds that high humidity often does its damage unseen, in walls, attics, and crawl spaces, so the glass is your early warning.

So the fix is almost always lower indoor humidity: below are what's normal, the moisture sources in the order to check them, what you can fix, and when to call a pro. The exception is fog between the panes of a double-pane window, a failed seal rather than humidity. If your air is too dry instead, see our humidifier guide.

First, Check What Is Normal

Find which surface is wet. On the room side of the glass, it's indoor humidity. On the outside, it's dew, which both makers say shows the glass is insulating well. Between the panes of a sealed unit, the seal has failed.

Some condensation is temporary. Both makers say a house that soaked up summer humidity can fog for a few weeks after the heat comes on. Pella says new building materials can do it through the first few heating seasons, and a sudden cold snap can do it briefly. A thin strip around the edge of the glass, which runs colder than the center, does not mean the window leaks air (Pella).

The colder it gets outside, the less humidity the glass can handle. In Pella's example, with the house at 70°F and 0°F outside, the inside glass sat at 43°F. At 40 percent humidity the dew point was 44°F and the window fogged; at 30 percent it was 37°F and the glass stayed clear. Both makers say condensation is more likely where January averages 35°F or colder.

Most Likely Causes

1. A humidifier set too high, portable or whole-house. The EPA says not to humidify above 50 percent, and if water condenses on windows, walls, or pictures, to move the humidifier, lower its setting, or run it less. ENERGY STAR goes lower: "In colder climates, during the heating season, humidity levels should be in the range of 30% to 40% RH to prevent window condensation."
2. Showers and cooking without the exhaust fan. Pella says a family of four can add 15 to 20 pounds of moisture a day from breathing, showers, cooking, and dishes. ENERGY STAR says kitchen and bath fans should vent directly outside and run during showers and cooking and for 20 minutes after. The Building America Solution Center, a Department of Energy program, says to duct them outside, not into the attic or crawl space.
3. Laundry drying indoors, or a dryer venting indoors. ENERGY STAR says to vent clothes dryers to the outdoors and, without a dryer, to dry clothes outdoors rather than hanging them inside; the Solution Center says indoor hang-drying adds a considerable amount of moisture. A gas dryer is a carbon monoxide question too: LG's manual says a gas dryer must be exhausted to the outdoors.
4. Plants, an aquarium, or a hot tub. Pella lists large aquariums and greenhouse, hot tub, and pool areas as moisture sources: cover large aquariums and keep doors to those spaces closed. It also names plants near a window as a reason one window fogs and others don't.
5. A new or newly tightened house. Pella says new windows, doors, roofing, or siding cut air leakage, and a tighter house holds more humidity, so windows can fog more after the upgrade than before. ENERGY STAR says certified windows resist condensation but can still get it in cold weather.
6. Closed blinds and heavy drapes. Pella says window coverings don't cause condensation but restrict the flow of room air over the glass, leaving it colder, and ENERGY STAR recommends window treatments that let air flow over the window.
7. A damp crawl space or basement. Andersen and Pella both recommend a vapor barrier over a crawl space's dirt floor; the Solution Center says it should cover the entire floor and be secured to the walls, with a tight-fitting lid on any sump basin. ENERGY STAR says better drainage around the foundation can lower basement humidity.
8. An unvented gas or kerosene heater. The EPA says combustion appliances produce water vapor and raise humidity unless vented outside, and it suggests choosing a vented space heater when you replace an unvented one. Follow the safety note above.

What You Can Fix Yourself

Measure first. A hygrometer, which the EPA calls a small, inexpensive meter sold at many hardware stores, tells you where you stand. Keep it in a main room, away from the bathroom and the humidifier, and aim for ENERGY STAR's 30 to 40 percent, its range for the heating season in colder climates.

Turn the humidifier down as it gets colder. Pella's ceilings, based on University of Minnesota studies with the house at 70°F:

On a whole-house humidifier with a manual control, AprilAire says to anticipate drops in outdoor temperature and adjust the setting in the morning for the night's expected low; its automatic controls use an outdoor sensor to do it for you. If the windows still fog, Pella says to turn the humidifier down or off.

Run the fans every time. Bath fan for the shower and 20 minutes after, range hood whenever you cook (ENERGY STAR), lids on the pots (Pella). Make sure each fan ends outdoors, never in the attic; our ice dam guide explains why. Check, too, that the dryer duct is attached and its outside flap opens while the dryer runs.

Open the blinds and move the air. Open drapes and shades by day (Andersen), don't cover or deflect warm-air registers (Pella), and run ceiling fans to move air (Pella).

Add a storm window or film. The Solution Center says an interior storm window reduces the chance of room-side condensation by providing a warmer surface, and ENERGY STAR says shrink-film kits reduce moisture buildup from condensation. The inside layer has to be airtight, though: if room air leaks past an interior storm, condensation can form on the glass behind it. For exterior storms, University of Maryland Extension says to seal the main window with caulk and weatherstripping first and to choose storm windows with weep holes. Our weatherstripping guide covers film kits and seals.

Dry out the basement. Extend downspouts, keep gutters clear, and grade soil away from the foundation (ENERGY STAR); if the space stays damp, run a dehumidifier (Pella). Our basement dehumidifier guide covers sizing.

Wipe up water and clean small mold spots. The EPA says to dry condensation quickly, and wet materials within 24 to 48 hours, to prevent mold. For mold on a sill or frame, it says to scrub hard surfaces with detergent and water, dry them completely, and not paint or caulk over mold; wear gloves, goggles, and an N-95 respirator, and never mix bleach with ammonia cleaners. On size, the EPA says: "If the moldy area is less than about 10 square feet (less than roughly a 3 ft. by 3 ft. patch), in most cases, you can handle the job yourself."

When to Call a Pro

• Fog between the panes. Pella says the insulating glass needs replacing, and Andersen says the window does; lowering the humidity won't clear it. Contact the window's maker or installer and ask about the warranty.
• A bigger mold problem. The EPA says: "If there has been a lot of water damage, and/or mold growth covers more than 10 square feet, consult EPA's Mold Remediation in Schools and Commercial Buildings." Hire a contractor with mold experience, and talk to a doctor first if you have health concerns.
• Condensation that persists after you lower the humidity. For a very tight house, Andersen suggests an air-to-air heat exchanger, or a fresh-air intake on a forced-air furnace: an HVAC contractor's job.
• Fuel-burning appliances after air sealing. ENERGY STAR says sealing can make naturally venting gas or oil appliances back-draft, so have a contractor test them before and after; the CPSC says to have the heating system inspected every year.

Prevent It

Check the hygrometer at the first cold snap. Before heating season, confirm that every bath fan, the range hood, and the dryer vent outdoors; clear gutters and extend downspouts; and set the humidifier by the chart above, lower as winter deepens. Open blinds by day, close storm windows for the season, and keep CO alarms working on every level. Our winter guide schedules the rest.

Frequently Asked Questions

Is condensation on the inside of windows bad?

A little fog after a sudden cold snap can be temporary, but Andersen calls winter condensation on the inside of windows a warning sign. Left alone, it can peel paint and damage frames (Andersen), feed mold (EPA), and signal moisture building up where you can't see it (Pella).

What humidity should my house be in winter to stop window condensation?

ENERGY STAR says 30 to 40 percent in colder climates during the heating season; the EPA says below 60, ideally 30 to 50. On the coldest days go lower: Pella's chart tops out at 30 percent when it's 0 to 10°F outside.

Why is there condensation between my window panes?

The seal on the insulated glass has failed, so lower humidity won't fix it. Andersen says older windows can lose their seal over time, which also makes them less energy efficient, and that the window needs replacing; Pella says the insulating glass does.

The Bottom Line

Water on the inside of your windows is a humidity reading you can see. Measure it, keep it at 30 to 40 percent while the heat is on and lower on the coldest nights, run the fans, vent the dryer outdoors, open the blinds, and dry out a damp basement or crawl space. Fog between the panes is a failed seal and a call to the window maker, and any unvented fuel-burning heater is a carbon monoxide question first.

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